定语从句There is a mountain( B )the top is always covered with s

关于whose引导的定语从句有三种情况,如果用你上面那个例子来说明的话,就是这样：There is a mountain whose top is always covered with snow.
由于whose引导的定语从句the + n+of which这种结构来替换,而在英语中为了平衡句子的结构,又常常将of which提前,构成of which+the +n的形式.因此这个句子又可以改为以下两种情况：
There is a mountain the top of which is always covered with snow.不过呢,这样在先行词那部分显得有些冗余,句子感觉不太协调,因此习惯将of which 提前变成：
There is a mountain of which the top is always covered with snow.
这就成了答案要想要的那的句子.
总之注意：
用of which 替代 whose 的时候一定要加上定冠词the,反之要去掉the.这道题top前面有个the 因此只选B 不可能选A
of which这里就是作定语,作top的定语,它和whose 是一样的,译为“谁的”,在定语从句中译作“（先行词）的……”这里的先行词是mountain,是山,因此就译作“山的顶部……”

of which 修饰the top
which指代 a mountain
即山的顶端

which 指代mountain 本来是the top of mountain 在定语从句中 of提前到which 之前

因为本句中括号后的句子（ the top is always covered with snow ）不完整，所以用关系副词。但很明显，此句中没有关系副词，所以要用关系代词 which 或 that，但在关系代词前需加介词。